The present invention relates to a crankshaft of an in-line four-cylinder engine.
In an internal combustion engine, a crankshaft and connecting rods convert linear movement of pistons into rotation. The crankshaft rotates at a high speed while receiving great load from the pistons through the connecting rods. Accordingly, the crankshaft is required to have a high rigidity. A crankshaft includes arms extending in radial directions in relation to the axis of a rotary shaft of the crankshaft, and crankpins that are coupled to the arms and eccentric from the rotation axis of the crankshaft. Such eccentric crankpins hinder smooth rotation of the crankshaft and produce vibration. To suppress such vibration, in the configuration disclosed in Japanese Laid-Open Patent Publication No. 5-26297, each arm has a counterweight in a section opposite to the part to which a crankpin is coupled, thereby keeping the balance of the entire crankshaft during rotation.
FIG. 3A is a front view illustrating the crankshaft of a typical in-line four-cylinder engine, and FIG. 3B is a cross-sectional view of the crankshaft.
As shown in FIGS. 3A and 3B, the crankshaft has five journals J1 to J5 supported by bearings provided in a cylinder block (not shown). The journals J1 to J5 are coupled to four crankpins P1 to P4 by arms A1 to A8, respectively. The crankpins P1 to P4 correspond to four cylinders, respectively. Counterweights W1 to W8 are provided at ends of the arms A1 to A8, respectively, to keep the balance of the crankshaft during rotation.
In the crankshaft shown in FIGS. 3A and 3B, the crankpins P2 and P3 of the four crankpins P1 to P4 are at the same rotational phase. Thus, the journal J3 between the crankpins P2 and P3 receives a particularly great eccentric load. This makes it difficult to maintain an oil film between the journal J3 and the corresponding bearing.
A flywheel for smoothing rotation (not shown) is attached to one end of the crankshaft in the axial direction. Therefore, torsional resonance having a node of vibration at a portion where the flywheel is provided is generated in the crankshaft. The torsional resonance causes the engine to vibrate. To suppress such torsional resonance, the thickness of the material may be increased in the entire crankshaft, thereby increasing the torsional rigidity. However, this configuration is unfavorable since the weight of the entire crankshaft would be increased.
As described above, the shape of a typical prior art crankshaft is not determined in consideration of the dynamic characteristics in the actual use. Particularly, phenomena caused by eccentric load such as local squeezing out of the oil film and the generation of torsional resonance are not taken into consideration. In this respect, prior art crankshafts have room for improvement.